1. Field of the Invention
The present invention relates to a control system for an automatic transmission of a vehicle and, more particularly, to a control system for performing a control to prevent the shocks which might otherwise accompany the change in ranges.
2. Related Art
In the automatic transmission for a vehicle, a neutral state or a park stage, in which no power is transmitted, is established by switching the ranges, as based upon the manual operation of the driver. In this case, the torque transmission states in a gear speed change mechanism constructing the automatic transmission are switched to cause shocks due to the slips of the frictional engagement elements and the change in the output torque.
In a control system for an automatic transmission disclosed in JPA-4-312266, for example, the reverse stage is inhibited according to the situation of the vehicle when the neutral range is shifted to the reverse range for the reverse running. Specifically, the automatic transmission, as disclosed in this publication is constructed such that an auxiliary transmission unit capable of switching high and low stages is connected to the input side of a main transmission unit for setting the reverse stage and a plurality of forward stages, and such that the auxiliary transmission unit is set to the high stage when the reverse stage is to be set. The reason why the auxiliary transmission unit is set to the high stage for the reverse stage is that the gear ratio of the reverse stage to be set in the main transmission unit is so high that the total gear ratio may be given a practically proper value by setting the auxiliary transmission unit to the high stage.
In the case of the shift from the neutral state to the reverse range, moreover, a control is usually made to reduce the engine torque so as to reduce the torque to be applied to the frictional engagement elements during the speed change. When this input torque reducing control cannot be executed, however, the frictional engagement elements to be applied for setting the reverse stage may be deteriorated in their durabilities by their slips, and they are protected by inhibiting the reverse stage.
When the shift is made from the state, where the vehicle is stopped at the reverse gear stage, to the neutral range or the parking range so as to continue the stopped state, shift shocks occur if the torque to be applied to the output shaft is abruptly reduced. Especially the frictional engagement elements to be applied for setting the reverse stage include those to be applied for establishing a predetermined forward stage or for making the engine braking effective, and their apply/release characteristics are set to those for a frequently used gear stage. In the prior art, therefore, there is not made the release control which is suited for the shift from the reverse stage to the neutral state. Nor is provided this means in the invention disclosed in the aforementioned publication. In the prior art, therefore, at a shift from a running range to a non-running range, i.e., at the shift from the reverse range to the neutral range or the parking range, the shocks may possibly be physically felt by the change in the output torque when the frictional engagement elements are abruptly released.
In the prior art, as described above, the oil pressure is equally controlled at all times at a manual shift from the running range such as the reverse range to the non-running range such as the neutral range. As a result, the draining rate of the oil pressure changes when the oil viscosity changes. When the oil viscosity rises, for example, the drainage of the oil pressure from the frictional engagement element to be released is delayed. This delay in the drainage may deteriorate the shocks.